Syrian rebels seized control of the Islamic State (IS) group's main supply route to Turkey, according to an activist group.

"Rebel factions and Islamists took control of the northeast of al-Rai," a town occupied by IS on the border between Syria and Turkey, the Syrian Observatory for Human Rights said on Thursday.

"This is the main and one of the last crossing points with Turkey."

Observatory director Rami Abdel Rahman told AFP that rebels entered al-Rai on Thursday following two days of clashes.

According to Abdel Rahman, IS still controls a crossing point further east, in the town of Halwaniyeh, but "al-Rai was where they mainly smuggled in jihadists, whereas Halwaniyeh is reserved for top commanders".

IS has suffered a string of setbacks in recent months, including the loss of the ancient city of Palmyra, east of Damascus, to pro-government forces in March.

A ceasefire that came into effect on February 27 has drastically reduced violence across Syria, but areas controlled by IS and al-Qaeda's affiliate Nusra Front were exempt.

Abdel Rahman said that IS had lost control of at least 18 villages in the northern province of Aleppo in recent days.

However, the Syrian state news agency Sana said on Thursday that the group had kidnapped more than 300 employees of a cement factory amid fierce fighting between government forces and IS militants in the town of Dmeir east of Damascus.

The town is divided between IS control in the east and rebel control in the west, but several key positions around it, including a military airport and a power plant, are still in government hands.

Abdel Rahman said that violent clashes had taken place near the airport and power plant with at least 20 Syrian army soldiers and 35 IS fighters killed.

Sana said "employees and executives of the al-Badia cement factory" were abducted, after local residents earlier reported that at least 250 workers at the plant had been missing since Monday.

"The company has informed the industry ministry that it hasn't been able to make contact with kidnapped individuals," Sana said.

A Dmeir resident told AFP on Thursday she could hear heavy shelling around the city and residents were not daring to leave their homes.

"We have no electricity, we have no water. There are people fleeing from the eastern districts to the west," she said, asking not to be named out of fear for her safety.

Since Syria's conflict erupted in March 2011, thousands of people have gone missing - many of them arbitrarily arrested by armed forces - across the country.

More than 270,000 people have been killed and millions have fled their homes.

UN-backed peace talks to bring an end to the conflict are set to resume next week in Geneva, with the latest target being April 13.

"I will be very much insisting and pushing for... a serious discussion on political transition" at the upcoming round, the UN's Syria envoy Staffan de Mistura told reporters in Geneva.

He said that before then he would complete a diplomatic tour, including stops in Damascus and Tehran, to "verify the international and regional stakeholders' position" in order to have "concrete results in the next round of talks".